1. Field of the Invention
The present invention relates to an oil seal for use in a gas spring, hydraulic shock absorber or the like device to provide a hermetic seal between a cylinder and a piston rod thereof.
2. Description of the Related Art
In a gas spring, a piston rod is reciprocally disposed in a cylinder. Within the cylinder is filled high pressure gas together with lube oil. Accordingly, at the end of the cylinder is provided a fluid seal or oil seal so as to provide a seal between the cylinder and the piston rod.
The oil seal has an annular seal lip extending radially inward so as to fit closely, at an inner circumferential surface thereof, the outer circumferential surface of the piston rod. In this instance, in order that the seal lip can prevent leakage of oil and gas assuredly, it needs to retain a predetermined contact pressure under which it is urged for contact with the piston rod. However, if it is tried to simply make higher the contact pressure of the seal lip, the contact area at which the seal lip is brought into contact with the piston rod is increased with increase of the contact pressure, thus causing the oil film covering the circumferential surface of the piston rod to be liable to be cut or removed by the seal lip at the time of movement of the piston rod relative to the oil seal. Thus, when the piston rod is out of lube oil, the resistance of the oil seal to movement of the piston rod increases while the lubrication at the seal lip becomes worse, thus deteriorating the durability of the oil seal.
From the foregoing reasons, an oil seal capable of solving such a problem has been proposed as is disclosed in Japanese utility model second publication (after examined) No. 7-36210.
This oil seal is shown in FIG. 5 and generally indicated by 1. The oil seal 1 includes an annular seal lip 2 having an inclined inner circumferential surface 3 which is inclined with respect to a center axis of the oil seal 1 when observed in a longitudinal section, i.e., the annular seal lip 2 has a conical inner circumferential surface 3. At the inner circumferential surface 3, the seal lip 1 is formed with a plurality of annular projections 4a to 4e which are arranged in sequence axially of the oil seal 1. The annular projections 4a to 4e have end portions brought into contact with the outer circumferential surface of the piston rod 5. Such an oil seal 1 can be higher in the contact pressure of each of the annular projections 4a to 4e but smaller in the continuous or consecutive contact surface area, thus allowing the oil film formed on the outer circumferential surface of the piston rod 5 to be hard to be cut or removed by the seal lip 2. In the meantime, in FIG. 5, indicated by 6 is a cylinder and by 7 is a metallic reinforcement ring attached to the oil seal 1.
However, in the above described oil seal 1, the annular projections 4a to 4e are formed so as to be simply of the same height above the inner circumferential surface 3. Thus, when the seal lip 2 is subjected to the high pressure within the cylinder 6, the annular projections 4a to 4e located axially more inside of the cylinder 6 receives more of the cylinder inside pressure. By this, the contact pressure of the annular projection 4a becomes larger as compared with those of the other annular projections. As a result, the annular projection 4a located axially more inside of the cylinder 6 is deformed axially outside of the cylinder 6 more largely and its contact area also becomes larger. For this reason, a problem of the prior art oil seal 1 is that when the cylinder inside pressure is high the annular projection 4a located axially more inside possibly cuts or removes the oil film on the outer circumferential surface of the piston rod 5.